Nearly 2.5 million patients worldwide suffer from End Stage Renal Disease (ESRD). To treat ESRD, patients are either subjected to a kidney transplant or undergo hemodialysis. Though acting as artificial kidneys, dialysis machines are not implanted in the body; instead, they require durable external access points to the body's circulatory system, often in the form of an arteriovenous fistula (AVF). An AVF is created via an artificial junction, or “anastomosis”, between an artery and a vein which is used to increase the volume of blood flow through the vein. Over time, the increase in blood flow volume increases the size of the vein.
As the circulatory system is typically understood, blood flows away from the heart through a series of arteries. Arteries branch off to even smaller vessels called capillaries, where nutrients such as oxygen are delivered to muscle tissues and cells. Thereafter, deoxygenated blood continues to flow through capillaries, and eventually returns to larger vessels called veins. Veins carry deoxygenated blood back to the heart and lungs, where the blood is reoxygenated and continues through the circulatory system.
In order to create an arteriovenous fistula, this process is short-circuited. There are a number of locations within the body where an arteriovenous fistula may be created, but for hemodialysis patients the most common location is on the non-dominant forearm. In order to create the fistula, the patient is generally put under anesthesia and a small incision is made to open up the patient's forearm in order to expose a superficial vein. The cephalic vein is tied off from blood flow and subsequently severed. The proximal (that is, the segment of the cephalic vein which maintains blood flow to the heart) is then sutured directly to the nearby radial artery and blood flow through the vein is resumed.
Because the normal capillary diffusion system is eliminated, blood flow through the cephalic vein is increased beyond what the vessel is accustomed to. In order to accommodate the increased blood flow, the size of the cephalic vein begins to expand over a period of weeks until the vein itself begins to bulge from under the skin of the patient, a process called AVF maturation. When the bulging vein has reached sufficient size, medical personnel implant dialysis needles into the vein such that dialysis machines can be connected to the patient's circulatory system.
The best hemodialysis vascular access is an Arteriovenous Fistula (AVF). In order to create an AVF, it is required to connect an artery with a vein. After the surgery 6-8 weeks are needed for fistula maturation. During the maturation the venous segment of the fistula is growing. The growing of the vein is triggered by the increased blood flow through the vein. The laminar flow of the blood through the fistula is responsible to initiate a cascade of events leading the fistula vein growth and fistula maturation. Conversely a turbulent blood flow will stimulate the vein to stenose preventing the fistula from maturing.
Unfortunately, there are a number of complications that may occur in the creation of an arteriovenous fistula. For example, the typical AVF requires 6-8 weeks to mature to a size and strength sufficient to support insertion of a dialysis needle. Further, 45-55% of AVFs fail to sufficiently mature, requiring the creation of a new arteriovenous anastomosis and a further 6-8 weeks of maturation time before a new fistula is created.
Many of the AVF created do not mature to a usable AVF. In US, the AVF maturation rate ranges between 45-55%. Most of the times the cause of nonmaturation is the presence of perianastomotic stenosis (narrowing of the vessel in the area of the surgical anastomosis) found to be present in 75% of nonmaturing AVF. The trauma of the surgery and the turbulent blood flow are believed to be responsible for the development of perianastomotic stenosis. The turbulent flow involved in the lack of fistula maturation might be created by the steep angle the vein is anastomosed to the artery. FIG. 1A illustrates a vein 31 formed into a curvature 23 attached to an artery 32. The attached vein 31 forms a steep anastomosis angle 36 approaching 90 degrees. FIG. 1B illustrates the degraded situation of the anastomosis of FIG. 1A after a few weeks. FIG. 1B illustrates a partially matured arteriovenous anastomosis D formed by an artery A and a vein C. There is a panastomosis B formed in the vein C. Note as well the steep angle 36 of the anastomosis D.
AVF developmental complications are generally attributed to increases in blood flow shear stress, i.e. turbulence, created due to the artificial nature of the arteriovenous anastomosis. As the blood flows through the artery and into the vein, it has been observed to eddy, or pool, instead of flowing smoothly through the anastomosis. This turbulent flow causes blood vessel stenosis—a narrowing of the vessel which limits the available flow rate. As stenosis occurs in the fistula anastomosis, the volume of blood flow is reduced, and the vessel may fail to stretch to the volume required to support a dialysis access port. It is therefore desirable to design a device which minimizes the amount of turbulent blood flow through the AVF anastomosis.